It is known to produce hollow bodies, having at least one undercut formed by a radially offset circumferential wall portion, in two operations immediately succeeding each other. In a first mold, constituting an injection mold, a preform is produced on a core by injection molding, and the core, carrying the preform, is introduced into a second mold, constituting a blow mold, in which the preform is inflated to form a hollow body having the undercut. To be able to finish-shape such hollow bodies, it has been proposed to divide the blow mold in an axial plane and, after blow molding, to move the two mold parts radially apart into an open position so that the hollow body can be removed laterally normal to the direction of separation of the mold parts and through the gap created between the mold parts. This, of course, is possible only when the separation gap of the blow mold is somewhat larger than the largest diameter of the hollow body.
Particularly in the case of multiple molds, where several blow mold rows lie side-by-side in the separating direction, this leads to an undesirably great space requirement since, between two adjacent rows, a free space, allowing the separating movement, must always be present, and the width of this space must be somewhat greater than the largest diameter of the hollow body. To avoid this large space requirement transversely to the blow mold axis, it has been proposed to perform the finish-molding after the radial moving apart of blow molds doubly or multiple divided along axial planes, in an axial direction. However, in this case also the disadvantage remains that the blow mold is separated all the way along at least two generatrices, which not only means a complicated, expensive mold and relatively large closing or locking forces, but also leads inevitably to burr formation on the hollow bodies along the mold joints. Hollow bodies with perfectly smooth generated surfaces cannot be produced in this manner.